1. Field of the Invention
The present invention relates to an image forming apparatus, and, more particularly, a device for controlling the surface potential of a photosensitive body used in an image forming apparatus, such as an electronic copying machine.
2. Description of the Related Art
In general, where photographs are copied by electronic copying machines, the resulting copied images will lack halftones, i.e., gray level. This is due to the surface potential of the photosensitive body.
Accordingly, an electronic copying machine has been developed which has a dedicated photo mode in which photographs are to be copied, as well as a normal mode in which originals other than photographs are to be copied. This type of electronic copying machine is constructed to make the surface potential of the photosensitive body in the photo mode lower than that in the normal mode, thereby improving the reproducibility of halftones.
Usually, in order to control the surface potential of the photosensitive body in the photo mode, an output value of a charging transformer is feedback controlled by means of a central processing unit (CPU).
With such an arrangement, however, a feedback control transformer and a feedback circuit are required, thus increasing manufacturing cost.
As described above, an electronic copying machine which feedback controls an output value of a charging transformer to control the surface potential of a photosensitive body has a drawback in that it is costly because of the need of a feedback control transformer and a feedback circuit.
A related invention is described in U.S. Ser. No. 466,586, filed on Jan. 17, 1990.